It is well known that an aqueous solution in contact with an untreated metal substrate can result in corrosion of the untreated metal substrate. Therefore, a metal article, such as a metal container for a water-based product, like a food or beverage, is rendered corrosion resistant in order to retard or eliminate interactions between the water-based product and the metal article. Conventionally, corrosion resistance is imparted to the metal article, or to a metal substrate in general, by passivating the metal substrate, or by coating the metal substrate with a corrosion-inhibiting coating.
Investigators continually have sought improved coating compositions that reduce or eliminate corrosion of a metal article and that do not adversely affect an aqueous product packaged in the metal article. For example, investigators have sought to improve the imperviousness of the coating in order to prevent corrosion-causing ions, oxygen molecules, and water molecules from contacting and interacting with a metal substrate. Imperviousness can be improved by providing a thicker, more flexible and more adhesive coating, but often, improving one particular advantageous coating feature is achieved at the expense of another advantageous coating feature.
In addition, practical considerations limit the thickness, adhesive properties and flexibility of a coating applied to a metal substrate. For example, thick coatings are expensive, require a longer cure time, can be esthetically unpleasing, and can adversely affect the process of stamping and molding the coated metal substrate into a useful metal article. Similarly, the coating should be sufficiently flexible such that the continuity of the coating is not destroyed during stamping and molding of the metal substrate into the desired shape of the metal article.
Investigators also have sought coatings that possess chemical resistance in addition to corrosion inhibition. A useful coating for the interior of a metal container must be able to withstand the solvating properties of a product packaged in the metal container. If the coating does not possess sufficient chemical resistance, components of the coating can be extracted into the packaged product and adversely affect the product. Even small amounts of extracted coating components can adversely affect sensitive products, like beer, by imparting an off-taste to the product.
Conventionally, organic solvent-based coating compositions were used to provide cured coatings having excellent chemical resistance. Such solvent-based compositions include ingredients that are inherently water insoluble, and thereby effectively resist the solvating properties of water-based products packaged in the metal container. However, because of environmental and toxicological concerns, and in order to comply with increasingly strict governmental regulations, an increasing number of coating compositions are water based. The water-based coating compositions include ingredients that are water soluble or water dispersible, and, therefore, cured coatings resulting from water-based coating compositions often are more susceptible to the solvating properties of water.
Epoxy-based coatings and polyvinyl chloride-based coatings have been used to coat the interior of metal containers for foods and beverages because these coatings exhibit an acceptable combination of adhesion to a metal substrate, flexibility, chemical resistance, and corrosion inhibition. However, epoxy-based coatings and polyvinyl chloride-based coatings have serious disadvantages that investigators still are attempting to overcome.
For example, coatings based on polyvinyl chloride or related halide-containing vinyl polymers, like polyvinylidene chloride, possess the above-listed advantageous properties of chemical resistance and corrosion inhibition, and are economical. However, curing a polyvinyl chloride or related halide-containing vinyl polymer can generate toxic monomers, such as vinyl chloride, a known carcinogen. In addition, the disposal of a halide-containing vinyl polymer, such as by incineration, also can generate toxic monomers. The generated vinyl chloride thereby poses a potential danger to workers in metal can manufacturing plants, in food processing and packaging plants, and at disposal sites. Disposal of polyvinyl chloride and related polymers also can produce carcinogenic dioxins and environmentally harmful hydrochloric acid. Government regulators, therefore, are acting to eliminate the use of polyvinyl chloride-based coating compositions that contact food, and thereby eliminate the environmental and health concerns associated with halide-containing vinyl polymers.
To overcome these environmental concerns, epoxy-based coating compositions recently have been used to coat the interior of food and beverage containers. However, epoxy-based coatings also possess disadvantages. For example, epoxy-based coating compositions are more expensive than polyvinyl chloride-based coating compositions.
Various patents disclose waterborne coating compositions for metal cans. In general, prior patents disclose coating compositions including waterborne thermoset resins for use as can coatings. The thermoset resins can be formulated with a crosslinking agent to provide crosslinked films during cure, as demonstrated by the resistance of the cured coating to the effects of organic solvents such as methyl ethyl ketone. The cured thermoset resins often do not have sufficient flexibility for use as can coatings.
Recently, waterborne phenoxy resins were disclosed as useful in coatings for metal cans. These waterborne phenoxy resins are high molecular weight thermoplastic resins that are difficult to process and are too expensive for practical commercial use. In addition, because these phenoxy resins are thermoplastic resins, cured coatings derived therefrom are not resistant to organic solvents, although the cured coatings often provide sufficient barrier properties to water-based compositions for use as can coatings.
Investigators, therefore, have sought a waterborne coating composition for the interior of food and beverage containers that retains the advantageous properties of adhesion, flexibility, chemical resistance and corrosion inhibition, and that is economical and does not adversely affect the food and beverages packaged in the container.
Investigators prefer a thermosetting coating composition because such compositions are easier to handle and provide better chemical resistance than thermoplastic coating compositions. A thermosetting coating composition also requires a crosslinking agent, generally a phenolic resin, an aminoplast, or a similar resin, in order to provide a cured coating having a sufficient molecular weight.
Prior investigators have studied waterborne epoxy resin-based compositions for application to metal substrates. Many of these investigators sought epoxy resin-based aqueous compositions that provide a sufficiently flexible cured coating such that the coated metal substrate can be deformed without destroying film continuity. Often, conventional epoxy resins provide a rigid cured film thereby making it difficult to impossible to coat the metal substrate prior to deforming, i.e., shaping, the metal substrate into a metal article, like a metal can. Coating a metal substrate prior to shaping the metal substrate is a standard industrial practice.
For example, Johnson et al. U.S. Pat. No. 4,954,553 discloses an aqueous coating composition comprising a carboxyl-bearing phenoxy resin and a resin that is soft in comparison to the phenoxy resin, like a polyester. The carboxyl-bearing phenoxy resin is prepared by grafting ethylenically unsaturated monomers to the phenoxy resin. The coating composition provides flexible cured coatings. Fan U.S. Pat. Nos. 4,355,122 and 4,374,875 disclose a waterborne phenolic composition wherein an ethylenically unsaturated monomer including a carboxyl group is grafted onto a phenoxy resin by standard free radical polymerization techniques, then the carboxyl groups are neutralized by a base.
Chu et al. U.S. Pat. No. 4,446,258 discloses an aqueous coating composition comprising: (1) the neutralized reaction product of an epoxy resin with a preformed addition polymer containing carboxyl groups, and (2) an acrylic or vinyl polymer, which is prepared either in situ or added to the composition, and which is different from the preformed addition polymer.
Evans et al. U.S. Pat. No. 4,212,781 discloses grafting an acrylic monomer or monomer blend to an epoxy resin to provide a polymeric blend including unreacted epoxy resin, an acrylic resin and a graft polymer of the acrylic resin and epoxy resin. Steinmetz U.S. Pat. No. 4,302,373 discloses a waterborne coating composition consisting essentially of the neutralized reaction product of a modified polyepoxide (e.g., an ester or ether) or a phenolic and a carboxyl-functional polymer.
Patel U.S. Pat. No. 4,963,602 discloses aqueous coating compositions including an epoxy resin, an acrylic resin, a phenoxy resin, a novolac resin, and a resol resin. Wu U.S. Pat. Nos. 3,943,187 and 3,997,694 disclose an organic solvent-based coating composition consisting essentially of a blend of an acrylic polymer having hard and soft segments and an epoxy resin. Salensky U.S. Pat. No. 4,638,038 discloses modified phenoxy resins wherein anhydrides or polycarboxylic acids are grafted onto a phenoxy resin. Spencer U.S. Pat. No. 5,296,525 discloses (a) the reaction product of an epoxy resin with a monomer having unsaturated groups, (b) wherein the reaction product of (a) then is reacted with a preformed carboxyl-functional polymer and a tertiary amine, (c) followed by reacting the reaction product of (b) with unsaturated monomers in an emulsion polymerization.
Other patents that disclose epoxy resins admixed with acrylic resins, or having acrylic resins grafted thereon, include Matthews et al. U.S. Pat. No. 4,247,439; Evans et al. U.S. Pat. No. 4,308,185; Wu U.S. Pat. No. 4,021,396; McCarty U.S. Pat. No. 4,444,923; Brown et al. U.S. Pat. No. 4,585,813; and Ting et al. U.S. Pat. No. 4,480,058.
Publications disclosing a water-based coating compositions including an epoxy resin and an acrylic resin include:
J. T. K. Woo et al., "Synthesis and Characterization of Water-Reducible Graft Epoxy Copolymers," J. Coat. Tech., 54 (1982), pp. 41-55; and PA1 R. N. Johnson et al., "Water-Borne Phenoxy Resins Low VOC Coatings with Excellent Toughness, Flexibility and Adhesion," presented at the Water-Borne and Higher-Solid Coatings Symposium, Feb. 3-5, 1988 in New Orleans, La. PA1 (a) a water-dispersible polymer prepared from PA1 (b) a fugitive base, like a tertiary amine; PA1 (c) a curing agent; and PA1 (d) a carrier comprising water and a volatile organic solvent. PA1 (a) about 5% to about 60%, by weight of nonvolatile material, of a water-dispersible polymer; PA1 (b) a sufficient amount of a fugitive base to render the water-dispersible polymer water dispersible; and PA1 (c) about 0.5% to about 25%, by weight of nonvolatile material, of a curing agent, like a phenolic resin or an aminoplast.
The above-identified patents and publications disclose waterborne coating compositions comprising an epoxy resin and an acrylic resin. The patents and publications do not disclose a waterborne coating composition comprising a water-dispersible polymer comprising an epoxy resin covalently linked to an acrylic resin by a linking compound having conjugated carbon--carbon double bonds or a triple bond.
The present coating compositions, after curing, demonstrate: (1) excellent flexibility; (2) excellent adhesion; and (3) excellent chemical resistance and corrosion inhibition.